In an age of ubiquitous networks, it is conceivable that sensors, each having radio communication functions, will communicate with each other by radio and establish a sensor network. Such sensor network will be required to provide a user at a remote site with access to sensing data collected automatically from the respective sensors.
However, in ordinary conventional radio communication systems, it has not been expected that a radio communication network is to be formed with small devices such as sensors, and a large amount of data have to be processed at the time of radio communication. Consequently, it is difficult to form a radio communication network with sensors.
Additionally, it is often the case that a small electronic device as a sensor is used in a place where a power supply line cannot be secured. In such a case, the small electronic device works on a solar battery, an accumulator (storage battery) or the like as a necessity. Therefore, the small electronic device is required to operate while consuming lower amounts of power.
Conventional radio communication systems in general were not developed in consideration of low electric power consumption. In this respect, the conventional systems are not suited to form a radio communication network with small electronic devices such as sensors.
As conventional radio communication systems which may have applicability to creating a radio communication network with small electronic devices such as sensors, there have been only such systems as follows:
(1) systems, in which respective sensor terminals simultaneously exchange path information at regular time intervals, as described in Japanese Patent Application laid open No. 2001-237764, “Multi-hop Radio Network and Radio Station”; and
(2) systems, in which not all sensor terminals establish a data transmission path but part of backbone terminals automatically set the path, and fixed radio communication is held between respective backbone terminals and sensor terminals.
According to the aforementioned “Multi-hop Radio Network and Radio Station”, each radio station obtains information on the number of hops from an accessible radio station. When direct access to a base station is available, the base station is selected as a connection destination radio station. In other cases, one accessible radio station whose number of hops can be made the minimum is selected as a high order connection destination radio station. Then, a signal received from a slave radio station is transferred to the high order radio station.
In the system (1) as described above, however, the frequency of transmission including unnecessary one increase, and therefore, it is difficult to reduce electric power consumption. For that reason, the system (1) is not suited to small devices such as sensors.
Besides, in the system (2) as described above, it is impossible to meet a requirement that data transmission paths are automatically set in such a manner as to collect data at one place (a base station) regardless of the placement of all devices.
There has been disclosed another conventional technique for forming a radio communication network with small electronic devices such as sensors in Japanese Patent Application laid open No. HEI9-331284, “Radio Relay System”.
According to the “Radio Relay System”, a message is relayed to its destination via electronic devices located between sending and receiving electronic devices to reduce the power required for transmission. This means that an electronic device that sends a message needs prior knowledge as to which electronic devices are present between it and an electronic device as the destination of the message. There is no problem when each of electronic devices is fixed in a position. However, if the electronic devices change their positions, that is, if the network structure is variable and data transmission paths to a base station have to be established independently, the radio relay system cannot be employed.
As described above, in the conventional radio communication system, there has not been proposed a data transmission path establishing method for establishing a data transmission path from each device to a base station independently, thereby forming a network with small devices which are required to consume lower amounts of power, such as sensors.